1. Field
The present disclosure relates generally to audio devices and systems, and more specifically, to a system and method of reducing click and pop noise in audio playback devices.
2. Background
In many audio systems, the output of an audio device is coupled to a speaker via a capacitor, typically referred to as a direct current (DC) blocking or alternating current (AC) coupling capacitor. Usually, the output of an audio device consists of an audio signal and an associated DC offset voltage. Prior to turning on the audio device, the voltage across the AC coupling capacitor is typically zero (0) Volt. When the audio device is turned on, the audio device charges the AC coupling capacitor to the associated DC offset voltage.
The charging of the AC coupling capacitor produces a rising voltage that typically has frequency components within the human audible range. These frequency components typically produce undesirable noise at the output of the speaker, which is typically referred to in the relevant art as “click and pop” noise. Similarly, when the audio device is turned off, the charge on the AC coupling capacitor decays producing a falling voltage that typically also has frequency components within the human audible range. Again, these frequency components produce undesirable click and pop noise at the output of the speaker. This is better explained with reference to the following example.
FIG. 1 illustrates a block diagram of an exemplary conventional audio system 100. The audio system 100 delivers an audio signal to a speaker 150 via an AC coupling capacitor CAC. In this example, the audio system 100 consists of a first operational amplifier OPA1, a second operational amplifier OPA2, and resistors R1A, R1B, R2A and R2B. The first operational amplifier OPA1 serves to amplify the input audio signal, which may be configured as a differential signal Vim and Vip. The second operational amplifier OPA2 is configured as a voltage-follower to generate a reference DC voltage Vref at the output of the first operational amplifier OPA1. This voltage Vref is typically set to Vdd/2 to optimize or improve the dynamic range of the audio signal at the output of the first operational amplifier OPA1.
The resistors R1A and R1B serve as input resistors to the first operational amplifier OPA1 from the perspective of the input audio signal Vim and Vip. The resistor R2B serves as an input resistor to the first operational amplifier OPA1 from the perspective of the reference voltage Vref generated by the second operational amplifier OPA2. The resistor R2A serves as a feedback resistor for the first operational amplifier OPA1.
Prior to the audio system 100 being turned on, the voltage across the AC coupling capacitor CAC is typically about zero (0) Volt. When the first and second operational amplifiers OPA1-2 are initially turned on via the EN1 and EN2 power inputs, the voltage across the AC coupling capacitor CAC begins to rise from zero (0) Volt towards the reference voltage Vref. Typically, the transitioning voltage has frequency components that lie within the human audible range. This typically produces an undesirable click and pop noise at the output of the speaker 150.
When the audio system 100 is turned off, the voltage across the AC coupling capacitor CAC decays from the reference voltage Vref towards zero (0) Volt. Similarly, the transitioning voltage typically has frequency components that lie within the human audible range. This also produces an undesirable click and pop noise at the output of the speaker 150.